1. Field of the Invention
The invention is mainly related to processes of industrial wastewater purification and in particular, to electrochemical processes of their treatment for removal of heavy metals, such as Fe, Cr6, Cr3, Cu, Zn, Cd and others. This wastewater is produced by enterprises using chromium compounds, non-ferrous metallurgy and electro galvanic coatings.
2. Description of the Prior Art
Methods for wastewater purification from chromium (Cr3 or Cr6— trivalent or hexavalent) and other heavy metals are well known, but many of those methods are not suitable for purification of large volumes of wastewater (millions of cubic meters per year).
These limitations of existing methods (such as ion exchange, membrane technologies, including ultrafiltration, reversed osmosis, chemical coagulation, coagulation with DC current, etc.) include:                a) low capacity of devices with periodic operation, when it is necessary to retain water in the reaction zone for sufficiently long period of time,        b) difficulties in fabrication and operation of these devices with dimensions over 100 m3,        c) high cost of needed reagents, for example, in ion-exchange technology,        d) extreme complexity of some methods, making them technologically vulnerable to provide continuity and trouble-free operation, for example in membrane ultrafiltration technology.        
Chemical methods of wastewater purification are not applicable today due to difficulty in reusing purified water in technological processes. Typically, conventional water purification methods provide utilization of sludge extracted from this water.
A large number of electrocoagulation methods and devices are known for wastewater purification from chromium and heavy metals. They use direct current with a current density on electrodes not less than 2.5/dm2 in fixed electrolytic cells, which work periodically (see e.g., U.S. Pat. No. 4,908,109, U.S. Pat. No. 4,917,782, U.S. Pat. No. 5,022,974, U.S. Pat. No. 5,094,757, U.S. Pat. No. 5,108,563, and U.S. Pat. No. 6,294,061.
Application of direct current during wastewater electrocoagulation has several disadvantages. These include:                1. Necessary application of powerful rectifying devices and big loss of current due to rectifying.        
2. The application of direct current in multielectrode stacks usually with small inter-electrode distance (10 mm) leads to uneven wear of the electrodes. Typically, the anode material dissolves and electroreduction products stick to the cathodes. This results in frequent short circuits requiring emergency downtime for changing the electrode stacks or transforming of partly worked stacks into secondary scrap. Uneven wear of electrodes during application of direct current makes these methods of metal electro-coagulation from wastewater not practical even for small galvanic plants.
To avoid sticking of electroreduction products to the cathodes, two previous patents suggest the use of cathodes with mobile electrodes or switching their polarity (U.S. Pat. No. 4,908,109 and U.S. Pat. No. 4,917,782). However, the problems of switching frequency are not resolved to make these methods practically feasible.
Electrocoagulation methods and devices are known for purification mostly from organic impurities, dyes and oils. Typically, the methods use alternating current or alternating current superimposed over direct current (U.S. Pat. No. 4,053,378 and U.S. Pat. No. 4,690,741).
Application of alternating current eliminates uneven wear (dissolution) of electrodes, typical for methods and devices for electrocoagulation based on the direct current. However, these methods have disadvantages caused by process periodicity and small volume of treated solutions, which impedes purification of wastewater by continuous flow.
Another disadvantage of the known methods is the use of noncommercial frequencies, i.e. variation of frequency changes with electrocoagulation period or application of high frequencies (z) with changing amplitude of positive and negative voltage impulses of different duration and form.
As a result, special frequency generators are needed for the above methods. Additionally, application of electrocoagulation devices with periodic operation for large volumes of water as well as for solution purification in continuous flow is unknown. The application of frequency generators for such electrocoagulation devices does not improve the technical and commercial effectiveness of electrocoagulation with respect to the application of rectifiers in electrocoagulation with direct current. This is due to a commensurable loss of electrical power in rectification and for the generation of different asymmetrical frequencies.
A method utilizing an electrocoagulator for wastewater purification from heavy metals is described in U.S. Pat. No. 6,077,416. According to this method, solutions are treated in an electroreactor, containing non-movable and movable electrodes with application of three-phase alternating current. The basic design of the device of this method is in the placement of movable electrodes (made of aluminum scrap pieces or shavings) in a perforated plastic container. A movable electrode is placed in the inter-electrode space formed by fixed steel electrodes, which are fed with two phases of three-phased current. The aluminum electrode is grounded and moves in the inter-electrode space between steel electrodes.
An electroreactor of any design based on a combination of steel stationary electrodes and movable aluminum electrode is filled with initial solution to be purified of heavy metals and organic impurities. A two-phase power supply is used to provide power to stationary steel electrodes. Due to the bipolarity of the movable aluminum electrode, a voltage appears between this electrode and steel electrodes. A voltage drop per cm of inter-electrode space is determined by the value of voltage applied to the steel electrodes and the distance between electrodes. The process of solution electrotreatment in the electroreactor is performed up to the moment, when a test sample shows that the separated contaminant (a heavy metal or organic compound) concentration level is below the level required by a standard or regulation. Then the treated solution is transferred to a separation filter for water clearance.
Steel electrodes are placed horizontally strictly facing one another in rectangular electroreactors. In a cylindrical reactor, all peripheral steel electrodes are also oriented in parallel relative to a sectional view of the central steel electrode.
Power supply for steel electrodes, i.e., voltage drop between central and peripheral electrodes, is the same. Therefore, all peripheral electrodes are connected in parallel to the same current phase. For the electrocoagulation process, any two phases for three-phased current are used at the same time: either 1-2, or 2-3, or 1-3. All three phases are used primarily for providing power to 3-phase pumps and engines.
The application of two phases of alternating current has one major disadvantage—only partial utilization of three-phase power supply for electroreactor. As a result—the purification process has unproductive loss of power resulting in decreased device efficiency due to imperfect electric power utilization.